Measurement of certain analytes in blood, such as pH and carbon dioxide, is an important aspect of the clinical care of patients. Previously, such measurements have been made using gas chromatography and other chemical methods. These methods are disadvantageous in that it is necessary to ship the blood sample to a clinical laboratory for analysis, which often results in a delay of an hour or more. Moreover, since the blood gases change rapidly, the shipping time may cause the results to be invalid. Furthermore, these known methods cannot be used for continuous in-vivo monitoring of blood.
It is known to optically measure certain analytes by using fluorescence intensity measurements. Although fluorescence intensity measurements are desirable in their simplicity, such measurements suffer from source fluctuations due to noise, drift and the like, and are subject to fluorophore bleaching, probe wash-out and background fluorescence. Further, if the media is turbid or colored, the intensity measurements will be greatly affected. Moreover, since intensity is a linear product of numerous factors, such as the amount of fluorophore in each state, the excitation intensity, the excitation and emission bandpass, the wavelength sensitivity of the detector, and the like, a complex set of calibration curves must be used to accommodate these factors.